Login flow working

tests/in_memory_music_service.ts

@@ -0,0 +1,35 @@
+import {
+  MusicService,
+  Credentials,
+  AuthSuccess,
+  AuthFailure,
+} from "../src/music_service";
+
+
+export class InMemoryMusicService implements MusicService {
+  users: Record<string, string> = {};
+
+  login({ username, password }: Credentials): AuthSuccess | AuthFailure {
+    if (
+      username != undefined &&
+      password != undefined &&
+      this.users[username] == password
+    ) {
+      return { authToken: "token123", userId: username, nickname: username };
+    } else {
+      return { message: `Invalid user:${username}` };
+    }
+  }
+
+  hasUser(credentials: Credentials) {
+    this.users[credentials.username] = credentials.password;
+  }
+
+  hasNoUsers() {
+    this.users = {};
+  }
+
+  clear() {
+    this.users = {};
+  }
+}